The present invention relates to a wet-processing method for combustion ashes of petroleum fuels, containing ammonium sulfate and a method of utilizing ammonia components recovered by the wet-processing method. More in particularly the present invention relates to a wet-processing method which comprises at least a preparing-step of a slurry of combustion ashes of mixing combustion ashes and water, a double decomposition step for ammonium sulfate, a recovering-step of free ammonia from a gypsum slurry containing free ammonia produced in the double decomposition step, and a gypsum separation step successively, which is excellent in an ammonia stripping efficiency from the gypsum slurry in the ammonia recovery step and in which the clogging in a stripping device is suppressed, as well as a method of utilizing ammonia components with an industrial advantage.
In various kinds of combustion furnaces (combustion devices) using petroleum fuels such as heavy oils and or emulsions, etc., for example, boilers of oil fired power stations and dust incinerators, ammonia is added in the combustion gases for preventing the corrosion of the combustion furnaces, caused by sulfate gases (S03) contained in combustion gases.
Accordingly, the combustion ashes collected and recovered by an electrostatic precipitator disposed to the downstream of a fire flue end thereof contain ammonium sulfate in addition to ashes mainly composed of unburnt carbon and heavy metals (Ni, V, Mg, etc.). For example, the following ingredients are contained in the combustion ashes recovered from a boiler using high sulfur-content heavy oils.
C: 10 to 80% by weight PA1 NH.sub.4 : 0.5 to 20% by weight PA1 SO.sub.4 : 20 to 60% by weight PA1 V: 1 to 5% by weight PA1 Ni: 0.3 to 2% by weight PA1 Fe: 0.3 to 2% by weight PA1 Mg: 0.1 to 8% by weight PA1 SiO.sub.2 : about 0.1% by weight PA1 a preparing-step of a slurry of combustion ashes by mixing the combustion ashes with water, PA1 a double decomposition step for ammonium sulfate, PA1 a recovering-step of free ammonia from a gypsum slurry containing free ammonia obtained in the double decomposition step, PA1 the gypsum slurry being flowed down from an upper portion of a packed column and air or steams being blown from a lower portion of the packed column to bring air or steams into counter-current contact with the gypsum slurry, thereby conducting stripping for ammonia, and PA1 a gypsum separation step. PA1 transporting a mixture of ammonia stripped in the ammonia recovery step and steams in the form of an aqueous solution to an ammonia separation device disposed in a battery limit of an ammonia supply device in an exhaust gas channel of a combustion furnace; PA1 mixing a gaseous ammonia separated and recovered by the ammonia separation device with air; PA1 heating the mixture; and PA1 supplying the resultant mixture into the exhaust gas channel of the combustion furnace. PA1 a solid-liquid separation step of removing solids from the slurry of the combustion ashes, PA1 a metal oxidation step of supplying ammonia and an oxidative gas to an aqueous solution formed by removing the solids from the slurry of the combustion ashes to oxidize vanadium, thereby obtaining an aqueous solution containing ammonium metavanadate, and PA1 a crystallization step for the obtained ammonium metavanadate, PA1 the above-mentioned three steps being incorporated between the preparing-step of the slurry of the combustion ashes and the double decomposition step of ammonium sulfate as defined in the first aspect, and PA1 a mixture of ammonia stripped in the ammonia recovery step and steams being transported in the form of an aqueous solution to the metal oxidation step. PA1 a metal oxidation step of supplying ammonia and an oxidative gas to the slurry of combustion ashes to oxidize vanadium, thereby obtaining a slurry containing ammonium metavanadate, PA1 a solid/liquid separation step of removing solids from the slurry containing ammonium metavanadate, and PA1 a crystallization step of the obtained ammonium metavanadate, PA1 the above-mentioned three steps being incorporated between the preparing-step of the slurry of the combustion ashes and the double decomposition step as defined in the first aspect, PA1 a mixture of ammonia stripped in the ammonia recovery step and steams being transported in the form of an aqueous solution and supplied to the metal oxidation step. PA1 the obtained aqueous solution is transported under pressure and a temperature retaining condition through the supply pipe and supplied to a crystallization vessel in the crystallization step. PA1 a crystallization device comprising the crystallization vessel and a cooler disposed to the outside thereof which are connected by means of a circulation channel is used, PA1 a slurry in the crystallization vessel in an amount more than that of the raw material solution supplied is circulated to the cooler, and PA1 a slurry substantially identical in the amount with that of the raw material solution supplied is withdrawn from the crystallization device. PA1 which method comprises: PA1 processing a gypsum-containing slurry (A) by the first solid/liquid separator, PA1 separating into gypsum (C) and a supernatant (B) containing nickel and/or magnesium hydroxide mixed from the gypsum-containing slurry (A) and containing 1 to 20% by weight of gypsum, calculated as solids, based on the hydroxides, and PA1 processing the supernatant (B) by the final solid/liquid separator to separate into a solid (D) and a waste water (E) not substantially containing the solid.
As a method of treating the combustion ashes, a lot of methods, referred to as a wet-process, for recovering valuable ingredients such as vanadium, which take a countermeasure for public pollution by a closed system, have been proposed. Specifically, there can be mentioned wet-processes, for example, as disclosed in Japanese Patent Application Laid-Open (KOKAI) Nos. 60-19086 and 60-46930, and Japanese Patent Publication (KOKOKU) Nos. 4-61709 and 5-13718 already proposed.
For example, the process described in Japanese Patent Publication (KOKAI) No. 5-13718 comprises (i) a first step of mixing combustion ashes and water to form a slurry, while optionally adding sulfuric acid to control the pH value to not more than 3, (ii) a second step of separating solids (unburnt carbon, etc.), (iii) a third step of heating the obtained liquid portion to a temperature of not less than 70.degree. C. and oxidizing metals while supplying ammonia and an oxidant to adjust the pH value to 7 to 8, (iv) a fourth step of separating deposits (iron sludges), (v) a fifth step of cooling the obtained liquid portion to a temperature of 40.degree. C., thereby depositing vanadium compounds (ammonium metavanadate), (vi) a sixth step of separating the deposited vanadium compounds, (vii) a seventh a step of adding calcium hydroxide or calcium oxide to the obtained liquid portion to deposit gypsum and metal (nickel and magnesium) hydroxides, and liberating ammonia simultaneously, (viii) an eighth step of recovering ammonia by stripping from free ammonia, and (ix) a ninth step of separating the obtained gypsum.
However, there are drawbacks in the ammonia recovery methods described in the above-mentioned publications, respectively.
Namely, the method disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 60-19086 is a method comprising complicated steps of precipitating gypsum from a slurry before stripping, and passing the resultant supernatant through a distillation column to strip an ammonia gas. Such a method is industrially disadvantageous, in which calcium compounds such as gypsum and magnesium hydroxide tend to be deposited on the inner wall surface of the distillation column and the operation thereof can not be continued for a long time.
Further, the method described in Japanese Patent Application Laid-Open (KOKAI) No. 60-46930 is a method comprising supplying steams from the lower portion of an aerating tank so as to heat to a temperature of not less than 80.degree. C. and supplying air from the lower portion thereof so as to strip ammonia. In this method, the stripping efficiency is poor because the contact between the slurry and air is insufficient, and continuous operation is difficult.
Further, the methods described in Japanese Patent Publication (KOKOKU) Nos. 4-61709 and 5-13718 comprise supplying a slurry containing a large amount of gypsum to a separation column from the upper portion thereof but no concrete example for the separation column is described.
Referring to the recovered ammonia, it is supplied to a fire flue of a boiler or a metal oxidation step for reutilization. However, since ammonia separated from each of the processes described above is a gasous mixture containing a water content (moisture) evaporated from the slurry and a large amount of air, pipelines of a large diameter are required for transporting to a place to be utilized. Further, since the water content contained therein is condensed into drains, a draining device has to be disposed at the midway of long distance pipelines, which is disadvantageous in view of maintenance.
As a result of the present inventors' earnest studies, it has been found that in a recovering-ammonia step of a wet-processing method for combustion ashes of petroleum fuels, by flowing down a gypsum slurry from the upper portion of a packed column and blowing air or stream from the lower portion thereof so as to bring air or stream into counter-current contact with the gypsum slurry, ammonia is stripped from the gypsum slurry, an ammonia stripping efficiency from the gypsum slurry in the ammonia recovery step of the wet-processing method is excellent and the clogging in the packed column is suppressed, that is, though it has been considered that the packed column can not be used for a slurry containing a large amount of solids because of a clogging problem since the packed column contains a large amount of packings and has a narrowed channel, the packed column in the wet-processing method can be used with no practical problem. On the basis of the above-mentioned finding, the present invention has been attained.